Exhaust heat recovery type combined cycle turbine plants have been proposed. These include a multi-shaft type combined cycle turbine plant including a plurality of gas turbines and a single steam turbine, as well as a single shaft type combined cycle in which one gas turbine and one steam turbine are coupled to the opposite ends of the shaft of a generator.
According to a conventional system of load control of the single shaft type combined cycle turbine plant, the generator output is controlled by adjusting the fuel flow quantity supplied to the gas turbine.
FIG. 1 of the accompanying drawing is a block diagram showing a conventional load control system of a single shaft type combined cycle plant comprising a compressor 1, a gas turbine 2, an electric generator 3 and a steam turbine 4, all mounted on the same shaft. Air compressed by compressor 1 is supplied to a burner 5 for burning fuel supplied thereto through a fuel adjusting valve 6, and combustion gas is supplied to the gas turbine 2. The combustion gas discharged from the gas turbine 2 is supplied to an exhaust heat recovering boiler 7 to generate steam which is supplied to steam turbine 4 via a steam adjusting valve 8. The steam discharged from the turbine 4 is condensed by a condenser 9, whereby the generator 3 is driven by gas turbine 2 and steam turbine 4.
The number of revolutions of gas turbine 2, etc. is detected by a rotation number (or speed) detector 10 and its output signal is compared with a speed set signal from a speed setter 11 by a comparator 12. The output of comparator 12 is proportionally integrated by an operational amplifier 13 and then applied to a fuel adjusting valve 6 via a servo-amplifier 14 to adjust the opening of the fuel adjusting valve 6. As a consequence, the quantity of fuel supplied to burner 5 of the gas turbine is adjusted for controlling the output of the gas turbine. Since the enthalpy of the steam generated by the exhaust heat recovering boiler 7 is determined by the enthalpy of the exhaust gas from gas turbine 2, the output of the steam turbine 4 is mainly determined by maintaining the steam adjusting valve 8 at a fully opened state or at a partial opening. Thus, the output of the generator 3 supplied to an electric power system is determined by the product of the sum of the outputs of the gas turbine and the steam turbine and the efficiency of the generator.
Thus, the operator observes the generator output detected by a load detector 15 and displayed on an indicator 16 and manually operates the speed setter 11 to control the load.
A load setter 17 and a subtractor 18 may be added for the purpose of automatically effecting the operations of the operator. In this case, a load set signal output by the load setter 17 and a load signal output by load detector 15 are compared in each other with subtractor 18, the output thereof varying the set value of speed setter 11 so as to bring the difference to zero, that is to make the load (generator output) equal to the set value of the load.
Among the methods of load control wherein a plurality of single shaft type combined cycle systems are assembled into one unit or plant, may be mentioned a method (mode A) in which the loads of all shafts are uniformly varied during operation, and another method (mode B) in which the plant load is varied by starting and stopping individual shafts.
Such single shaft type combined cycle systems have a load variation rate characteristic similar to that of a conventional steam electric power generating plant so that there is a disadvantage that in mode B, while running with more than two shafts, the load variation rate as a single unit is smaller than that of mode A.
For example, in a plant having a capacity of 500 MW and comprising five single shaft type combined cycle systems each having a capacity of 100 MW, where the load variation rate is 5%/minute above 50% load and 3%/minute below 5% load, the load variation rates under various running conditions are shown in the following table.
______________________________________ Mode A Mode B Uniform load Plant load variation caused by variation in start and stop of shafts running shafts Load increase Load decrease Less Larger caused by start- caused by stopp- than than ing one shaft ing one shaft Num- 50% 50% Load of Load of Load of Load of ber of load load started started stopped stopped runn- per per shaft is shaft is shaft is shaft is ing one one less larger larger less shafts shaft shaft than 50% than 50% than 50% than 50% ______________________________________ 1 0.6 1 0.6 1 1 0.6 2 1.2 2 0.6 1 1 0.6 3 1.8 3 0.6 1 1 0.6 4 2.4 4 0.6 1 1 0.6 5 3.0 5 0.6 1 1 0.6 ______________________________________
As can be noted from this table, in mode A, when five shafts are running under the same load, the plant load variation rates of 3%/min. under a load of less than 50% and 5%/min. under a load greater than 50% can be obtained, whereas in mode B, the plant load variation rate is extremely small.
FIG. 3a shows a load variation when the load is decreased from 100% (500 MW with 5 shafts) to 60% in a 500 MW plant including five combined cycle shafts each having a capacity of 100 MW. In this case, when two shafts are stopped while the other three shafts are maintained at 100% load, the load varies as shown by dotted lines. Thus, the load variation rate of the plant is decreased.